I have noticed that there are more than one type of SABER (a hard blade and a flexible one), which one is really drived for Tai Chi Chuan ?I also found two types of handle, one with a rounded guard (as in katana sword) and other with a cross stick guard (as an arabian sword). As I am just starting to study DAO,so I really don't know how this differences should influence in the form exectution.

The basic idea is that the dao or saber has only one sharp edge and is slightly curved. Jian or sword by contrast is straight and both edges are sharpened. Sabers come in considerable variety. Mostly we see a fairly thick and rigid one often referred to as Mongolian broadsword. Somewhat rarer in the US are the ones the Yangs use, shaped somewhat like an old-fashioned butter knife with an s-shaped hand-guard. Even these come in various levels of cost, thickness, weight, sturdiness and detail. Whatever you use should be balanced and have a saber tassel or cloth. I recommend you consult with your teacher about buying equipment like sabers and swords.